The invention relates to a method for operating an internal combustion engine wherein the fuel reaches at least one combustion chamber via at least one fuel-injecting device which includes a piezo actuator. In the method, the drive energy, which is required for a specific stroke of the piezo actuator, is determined from at least one function which couples the stroke, the drive energy and the operating time of the piezo actuator to each other.
Such a method is disclosed in U.S. Pat. No. 6,328,019 (corresponds to German patent publication 198 48 950). Here, an internal combustion engine is operated with fuel injection valves which include piezoelectric actuators. These actuators, in turn, include a piezo stack, which is changeable in length under the action of an electric control voltage and operates on a valve member. The change of length is longer or shorter depending upon the magnitude of the control voltage. The relationship between the length change (stroke) and the control voltage is, however, dependent upon deterioration. For this reason, the suggestion is made in U.S. Pat. No. 6,328,019 to compensate the effects of deterioration of the piezo actuator via a sensor detection of the duration of operation measured in hours.
For this purpose, sensors detect the instantaneous use and operating conditions of the engine in which the fuel injection valve having the piezo actuator is utilized and, in dependence upon the signals of the sensors, a computer supported parameterization of the course of expansion is carried out. In summary, a function is formed which couples the stroke, the drive energy and the operating time of the piezo actuator to each other.
These measures function to meter the fuel quantity which is intended to reach the combustion chamber of the engine with the greatest possible accuracy. The fuel quantity flow which exits through the fuel-injecting device is influenced, inter alia, by the stroke of the piezo actuator. A precise metering of the fuel in an injection makes possible an optimization of the emission performance of the engine and of the consumption of fuel.
Notwithstanding the provided measures, it was, however, determined that the emission performance and the fuel consumption of the engine deteriorate over the service life of the engine.
It is an object of the invention to improve a method mentioned initially herein so that the emission performance and the fuel consumption are optimal during a longest possible duration of operation of the engine.
The method of the invention is for operating an internal combustion engine wherein fuel reaches a combustion chamber of the engine via a fuel injection device which includes a piezo actuator operating over a running or operating time (t) and for which a drive energy (U) is required for a specific stroke (h). The method includes the steps of: determining a function which couples the stroke (h), the drive energy (U) and the operating time (t) for a typical collective load (L1, L2) to which a piezo actuator can be subjected; and, utilizing the function to determine the required drive energy (U) for the specific stroke (h).
According to the invention, it has been determined that the characteristic line of a piezo actuator, that is, the interrelationship between the drive energy and the stroke is not only dependent upon the operating time (time span, number of actuations, et cetera) but also on the collective load to which the piezo actuator is subjected. Under the term xe2x80x9ccollective loadxe2x80x9d, the trace of at least an external load is understood to which the piezo actuator can be subjected. If, throughout its service life, only low external loads act on a piezo actuator, then, after a specific operating time, another drive energy is required in order to effect a specific stroke of the piezo actuator than in the case in which comparatively large loads act on the piezo actuator during the elapsed time span.
The typical collective loads are known for most applications of the fuel-injecting device, that is, of the piezo actuator built thereinto. If the collective load, which is typical for the specific use, is also considered in the determination of the function which couples the stroke, the drive energy and the operating time to each other, then one obtains a function with which one can determine the drive energy, which is required for a specific stroke of the piezo actuator, in dependence upon the operating time with very high precision. This function is then for the specific use of the fuel-injecting device, that is, the piezo actuator.
In this way, it is ensured that the fuel reaches the combustion chambers of the engine with high precision over the entire service duration of the piezo actuator and of the fuel-injecting device. In this way, the advantage of the invention of an optimization of the emission performance and of a reduction of the fuel consumption is achieved for engines operated in this way. It is understood that the invention can be applied equally to gasoline engines and diesel engines as well as to internal combustion engines having intake manifold injection or direct injection.
In a first embodiment, it is suggested that the collective load is typical for the use of a piezo actuator in a specific internal combustion engine and/or for the use of an engine under specific conditions. Accordingly, the typical collective loads are different, for example, for engines having different numbers of cylinders and/or with different arrangements of the cylinders. The collective loads of diesel engines are different than gasoline engines. Also, the way the fuel-injecting device is mounted on the engine block of the engine has an effect on the collective load.
Furthermore, collective loads are distinguished from each other depending upon whether the engine is utilized, for example, in a passenger automobile or in a truck or a locomotive or in an aircraft. Also the following can have effects on the typical collective load: the area of use and the corresponding road conditions; a chassis of a vehicle in which the engine is mounted; or the nature of the motor vehicle (especially sports car or limousine). The differentiation suggested here increases again the precision with which the fuel reaches the combustion chambers of the engine.
It is also possible that a plurality of functions is present. The functions apply for different collective loads and an actual collective load is determined by means of sensors during operation of the engine and, based on this actual collective load, that function is selected from the plurality of functions whose collective load is closest to the actual collective load. In this further improvement, one can react flexibly to a deviation of the actual collective load from the originally assumed collective load. In this way, always that function is available for the computation of the drive energy, which is needed for a specific stroke, during the entire service life of the engine. With this function, the effects of deterioration of the piezo actuator are considered in the best possible manner while considering the actual collective load to which the piezo actuator was subjected. Here too, the emission performance of the engine is improved and the fuel consumption reduced.
It is especially advantageous when the collective load includes a typical trace of the temperature of the piezo actuator and/or a typical trace of a drive time of the piezo actuator and/or a typical trace of accelerations to which the piezo actuator is subjected. Such collective loads include those parameters which have the largest effects on the deterioration characteristic of the piezo actuator. If a function is used for the determination of the drive energy, which initially had been determined for a collective load of this kind, then the deterioration characteristic of the piezo actuator is very accurately reflected.
In an advantageous configuration of the invention, it is suggested that a function be used, which was set up for at least a standard stroke of a piezo actuator, and that the drive energies for the strokes of the piezo actuator be derived from this function, which deviate from the at least one standard stroke. This can, for example, take place via corresponding reduction factors. In this way, storage space and computation time is saved which operates favorably on the cost in the realization of the engine.
It is also possible that the function includes a characteristic field specific for a collective load into which the desired stroke and the operating time is fed in and which outputs the needed drive energy therefor. This requires more storage space and possibly also more computation time but increases the precision of the determination of the drive energy needed for a specific stroke.
It is especially advantageous in the method of the invention when the functions were determined empirically. For this purpose, an internal combustion engine can be mounted on a test stand with the engine being equipped with a fuel-injecting device having a piezo actuator. During the operation of the engine, the engine is subjected to different external loads and to different courses of the load. However, it is also possible to mount the piezo actuator directly, for example, on a device for generating vibrations and to record the deterioration characteristic of the piezo actuator for a specific trace or course of the external loads. For another course of the external loads, the same experiment must be carried out with a new piezo actuator.
The result of the empirical determination of the function then provides more information when the piezo actuator, which is used to empirically determine the function, is first predeteriorated. In this way, the same start conditions are provided for the piezo actuators, which improves the result and therefore leads to a still better metering of the fuel into the combustion chambers of the engine when utilizing the function.
The invention also relates to a computer program, which is suitable for carrying out the above method when it is executed on a computer. Here, it is especially preferred when the computer program is stored on a memory, especially on a flash memory.
The invention also relates to a control apparatus (open loop and/or closed loop) for operating internal combustion engines. In order to optimize the emission performance of the engine and to reduce fuel consumption, it is suggested that the control apparatus include a memory on which a computer program of the above kind is stored.
The subject matter of the invention is also an internal combustion engine with at least one fuel-injecting device via which the fuel reaches at least one combustion chamber and which includes a piezo actuator and includes a control apparatus, which determines the drive energy, which is needed for a specific stroke of the piezo actuator, from at least one function which couples the stroke, the drive energy and the operating time of the piezo actuator with each other.
In order to improve the emission characteristic of the engine and to reduce the fuel consumption, it is suggested that the control apparatus determine the drive energy by means of a function which was determined initially for a typical collective load to which the piezo actuator can be subjected.
It is especially advantageous when the internal combustion engine includes a control apparatus of the above kind.